The present invention concerns chemical compositions for use in forming porcelain enamel coatings on aluminized steel. More particularly, the present invention concerns compositions for use in forming adherent, brilliant white, chemically and mechanically resistant, porcelain enamel coatings on aluminized steel.
Aluminized steel is widely used in automotive, appliance, building material, cookware, and other applications where rust resistance is required. In many of these applications, it is highly desirable to apply an adherent, bright white, chemically and mechanically resistant, porcelain enamel coating to the aluminized steel.
There are many conventional white porcelain enamel compositions known in the art for use on steel. However, these conventional porcelain enamel compositions, which are typically fired in 780xc2x0 to 950xc2x0 C. range, are not suitable for use on aluminized steel because they mature at temperatures exceeding the melting point of aluminum (660xc2x0 C.).
In an effort to apply porcelain enamel coatings to aluminized steel, resort was made in the past to special enamels which had been developed for use on aluminum and alloys of aluminum. However, use of these special enamels, which could be fired in the 530xc2x0 to 570xc2x0 C. range, was not particularly successful due in part to incompatible thermal expansion properties as between the special enamels and the aluminized steel. The linear coefficient of thermal expansion of aluminum, for which these special enamels were developed, is nearly twice that of aluminized steel.
Examples of prior art patents include U.S. Pat. No. 3,061,449, issued to Hoffman, which discloses a porcelain enamel composition for use on aluminized steel which contains significant levels of PbO. Although this enamel composition exhibited compatible thermal expansion properties for use on aluminized steel, it did not produce light colored opaque enamel surfaces. Pigments or other coloring agents had to be added to the composition according to Hoffman during milling in order to achieve a light color. It is generally desirable to achieve opacity in an enamel by having the glass frit component form opaque crystals on firing (self-opacifying), rather than by adding pigments or other coloring agents to the enamel system during milling, because self-opacifying enamels generally have superior luster, hiding power, and whiteness properties.
U.S. Pat. No. 5,266,357, issued to Preuss et al., discloses a process whereby a porcelain enamel coating can be applied to aluminized steel by: (I) applying an enamel slip composition containing a low-temperature enamel frit (passing into glass flux at less than 600xc2x0 C.), a mill addition of high-melting inorganic oxidic powder, and a mill addition of copper or iron oxides; (II) drying the slip coating; and (III) firing at temperatures in the 8000 to 850xc2x0 C. range. Although enamels applied to aluminized steel according to this process displayed improved adhesion and resistance properties as compared to the art at the time, light colored opaque enamel surfaces could not be produced by this process, thus requiring an additional second application or top coat of a white enamel composition in order to achieve a light colored enamel surface.
Accordingly, a composition is needed for use in forming an adherent, brilliant white, chemically and mechanically resistant porcelain enamel coating on aluminized steel. Such a composition should include a self-opacifying glass component, and should avoid use of toxic materials such as, for example, PbO, and should be able to be applied in only one coating layer.
The present invention is directed to a chemical composition for use in forming an adherent, brilliant white, chemically and mechanically resistant, porcelain enamel coating on aluminized steel. The composition according to the present invention includes a novel glass component and optional mill additions. The glass component includes by weight of from about 20 to about 40% SiO2, from about 20 to about 40% TiO2, from about 8 to about 20% Na2O, from about 0 to about 20% B2O3, from about 2 to about 15% K2O, from about 1 to about 11% Li2O, from about 0 to about 8% Sb2O3, from about 0 to about 5% P2O5, from about 0 to about 4% BaO, from about 0 to about 3% MgO, and from about 0 to about 3% Al2O3. The glass component may comprise one or more glass frits so as to provide the overall stated composition. The glass component is capable of self-opacifying on firing at temperatures well below the melting point of aluminum.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
The novel enamel composition of the present invention is particularly well-suited for use on aluminized steel such as that typically used in automotive, appliance, sanitary ware, building material, cookware, and other applications where rust resistance is required. Other uses will be apparent to those skilled in the art.
The novel porcelain enamel composition of the present invention preferably comprises from about 85% to about 100% by weight of the glass component. The glass component of the enamel composition may comprise one or more glass frits. Preferably, the glass component provides a composition as follows:
The glass frit or frits that comprise the glass component may be prepared utilizing conventional glass melting techniques. A conventional ceramic refractory, fused silica, or platinum crucible may be used to prepare the glass frit. Typically, selected oxides are smelted at around 1250xc2x0 C. for 30 minutes. The molten glass formed in the crucible is then converted to glass frit using water-cooled rollers and milling equipment. It will be appreciated that the step of producing the glass frit is not per se critical and any of the various techniques well-known to those skilled in the art can be employed.
The enamel composition according to the present invention is suitable for application to an aluminized steel substrate using conventional techniques such as, for example, either wet or dry application processes. Suitable wet application processes include wet spraying, wet electrostatic application, dipping and flow coating, which are well-known, and electrophoretic application. The disclosures of U.S. Pat. Nos. 5,002,903, 4,085,021 and 3,841,986 are incorporated herein by reference for their teachings relative to the electrophoretic application of porcelain enamel coating compositions. Dry application processes, such as for example a standard corona charging gun, are also well-known. The disclosures of U.S. Pat. Nos. 4,110,487 and 3,928,668 are hereby incorporated by reference for their teachings relative to the dry or electrostatic application of porcelain enamel coating compositions. When employing a wet application or technique, it is generally preferred that the aluminized steel be degreased prior to the application of the enamel.
It will be appreciated that the glass frit component may be used in combination with various mill additions. It will also be appreciated that the mill additions will vary depending upon the specific application conditions being utilized. For dry applications, the glass component may be used in combination with one or more conventional organopolysiloxanes. Such organopolysiloxanes are commonly employed in enamel compositions that are designed for dry application. For wet application processes, the glass frits may be milled in conjunction with other mill additions such as sodium molybdate, molybdenum trioxide (molybdic acid hydride/ammonium polymolybdate), sodium silicate, quartz, bentonite, etc. to produce a suitable composition. Also, it will be appreciated that depending upon the particular application one or more pigments or coloring agents may be included in the enamel composition. As is well-known in the art, there is a wide range of other acceptable mill agents or components that may also be utilized in the present invention to produce the desired resultant product. However, it will be appreciated that the use of certain additions may have an undesirable effect on the properties of the resultant enamel. Generally, it is best to avoid additions that contain any form of chlorine.
The resultant product preferably provides a coarseness of 1% by weight through a 325 mesh sieve (45 microns). However, it will be appreciated the milling fineness may be altered to meet the requirements of the specific application process being utilized or the desired final properties of the enamel.
Whether applied by wet or by dry processes, the coating composition of the present invention may be applied at a rate of from about 250 to about 450 grams per square meter so as to provide a coating thickness of from about 2 to about 8 mils subsequent to firing. The coating thickness is not critical, and can be varied based upon the particular application or need or opacity required.
Articles that have been coated with the coating composition of the present invention may be fired in a conventional manner using conventional firing equipment. Firing is generally conducted in an air convection furnace at a temperature of from about 560xc2x0 C. to about 600xc2x0 C. for a period of from about 6 to about 15 minutes. However, it will be appreciated that depending upon the specific application and composition of the glass component, a longer or shorter firing period could be used.
It will be appreciated that products coated with enamel compositions made in accordance with the present invention have a brilliant white opaque finish. The fired appearance of the final coating is smooth and uniform and substantially free of ripples or curtaining. Additionally, the enamel coatings made in accordance with the present invention are color stable, resistant to chemical attack (acid resistance), and display good mechanical resistance properties. Further, it will be appreciated that the enamels of the present invention may also be used on aluminum, of course, the thermal expansion of the enamel system may need adjustment to form a suitable bond on aluminum.
The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims: